Re: Two different domains in one Office 365 tenant

Hi,

There can be only one Azure AD Connect instance for a single Azure tenant. This means, you have to use one AAD Connect instance for both companies, if you want to go single tenant.

Azure AD Connect supports connecting multiple forests to a single Azure AD tenant. A server that runs Azure AD Connect does not have to be joined to any domain locally, however, it must be able to access domain controllers in both forests.

In some cases, you can choose to place the Azure AD Connect server in a (DMZ), especially if you do not have a direct network connection to all forests that you would like to include in the synchronization.

If you need more information, you probable should tell what is your goal and how both companies must work together.

Re: Two different domains in one Office 365 tenant

1. Could you migrate these two companies to a single domain (companyC.com)

2. If so, could you run Exchange hybrid until the migration is complete, for example, Joe from Company B is migrated to O365 he now has (joe@companyC.com), but Beth also from Company B has not been migrated, so she's still (beth@companyB.com). Mary from Company A is migrated to O365 so she now has (mary@companyC.com), but Tom also from Company A has not been migrated, so he's still (tom@companyB.com).

3. Could you migrate these two companies to multiple domains under a single tenancy, for example, company A stays companyA.com and company B stays companyB.com in O365, but under a single tenancy structure where administration, billing, and usage are tracked under a single account.

Re: Two different domains in one Office 365 tenant

1: yes you can

2: this also works fine (i've done this for a customer with 8 different source forests)

3: Microsoft will consider the tenant as a single customer, so you'd have to figure out some way to allocate license costs to the different companies. In addition, administrators will be able to administer ALL users (depending a little on setup), so you'll need some governance on that.